1. Field of the Invention
The present invention relates to a mirror rotation angle detection mechanism employed for the mirror rotation type tracking actuator and others of a high-speed access photoelectro-magnetic disc driving apparatus.
2. Related Background Art
In recent years, it has increasingly become necessary to detect precisely at a low cost the mirror rotation angle of a mirror rotation type tracking actuator in seek in order to speed up the access of a photoelectro-magnetic disc driving apparatus.
A conventional apparatus for this purpose is structured as shown in FIG. 1. In FIG. 1, the light ray 8 from an optical head is reflected by a mirror 1. The mirror 1, a mirror holder 2, and a coil 3 are rotatably held by a supporting member (not shown) to rotate in the plane of the drawing with the reflection point C of the light ray 8 on the surface of the mirror 1 as its substantial center. The coil 3 is wound around the mirror holder 2. A magnet 4 and a yoke 5 form a magnetic circuit. The mirror 1 is driven to rotate together with the mirror holder 2 and coil 3 when a current is applied to the coil 3. Photocoupler 63 and 64 are both reflective and output signals in accordance with the distance to a reflection surface 2a arranged on the mirror holder 2 by utilizing the property that the reflective luminous energy varies by distance. The reflective photocoupler 63 and the reflective photocoupler 64 are located relative to the mirror 1 at positions opposite to each other, and each of the output signal 63a and output signal 64a is inputted into a differential amplifier 7. The differential amplifier 7 picks up the differential between the output signal 63a and output signal 64a respectively of the reflective photocoupler 63 and photocoupler 64. The said differential is the difference between the two luminous energies, i.e., the difference in distances between the mirror surface 2a and the reflective coupler 63 and the reflective coupler 64, and is the rotation angle signal of the mirror which is in the plane of the drawing with the point C as its center.
The performance of the above-described conventional structure is limited by a number of factors. For example, the structure is subject to certain vibration modes, including, for instance, a vibration mode which causes the mirror holder 2 to have translational motion in the direction indicated by arrow A as shown in FIG. 1, and a rotational vibration mode with the center line B of the mirror 1 as its center. Also, the reflective photocoupler 63 and reflective photocoupler 64, which function as rotation angle sensors, do not detect the rotation angle of the mirror 1 directly. In practice, the reflective photocouplers detect the distance between the mirror holder 2 and the reflection surface 2a. This limits detection accuracy because, in manufacturing, the reflection surface 2a cannot be made exactly parallel with the reflection surface of mirror 1. Furthermore, differences in the sensitivities of the reflective photocouplers 63 and 64 limit detection accuracy.
As an example, if there occurs the vibration mode which results in the translational motion in the directions A and further, the translational motion is deviated, the waveforms of the output signals of the differential amplifier 7 are influenced, thus making it impossible to perform a highly precise detection of the rotation angles. Also, when the rotational vibration mode is generated which has the center line B as its center, the waveforms of output signals of the photocoupler 63, photocoupler 64, etc. are not equally affected if the manufacturing precision of the reflection surface 2a of the mirror holder 2 or the structural precision of the supporting member is not extremely high. Then, a highly precise detection of rotation angles cannot be performed.